1. Field of the Invention
The present invention relates to an improvement of a method of producing Josephson elements of the tunneling junction type having two superconductor metal films and a thin insulating film between the superconducting metal films.
2. Description of the Prior Art
FIG. 1 is a perspective view of such a Josephson element of the tunneling junction type. In FIG. 1, a the first electrode (i.e. a base electrode) 1 and a second electrode (i.e. a counter electrode) 2 are made of superconducting materials and are a few thousand angstroms thick. A thin insulating film 3 is made of oxide, is 10.about.50 angstroms thick and serves as a tunneling barrier.
The above-mentioned Josephson element has a current-voltage characteristic as illustrated in FIG. 2. In FIG. 2, I.sub.C represents the critical current, i.e. the maximum current which can flow at the zero voltage. In the case where a logic circuit is composed of Josephson elements, it is necessary to decrease or eliminate the difference between the critical currents of the Josephson elements. The critical current I.sub.C of a Josephson element is dependent upon the thickness and the area of the oxide film 3 of the Josephson element. In particular, the critical current I.sub.C depends exponentially upon the oxide film thickness. Accordingly, in order to obtain a predetermined value of the critical current I.sub.C, the oxide film formation must be accurately controlled to make the oxide film thickness a predetermined thickness.
For example, according to a conventional method of producing a Josephson element utilizing a conventional lift-off method, a photoresist is applied on a substrate (not shown in FIG. 1). The substrate is a single crystalline silicon substrate, a glass substrate, or a single crystalline silicon or glass substrate with a ground plane (e.g. an Nb film) and a dielectric film deposited thereon. The photoresist is exposed to light through a patterned mask, and then developed. As a result, the unnecessary part of the photoresist is removed and then the substrate with the remaining photoresist is cleaned with water. A superconducting material film is deposited on the substrate and the remaining photoresist by a conventional method, e.g. an evaporation method or a sputter deposition method. Then, the remaining photoresist and a part of the deposited film lying on the photoresist are removed at the same time by using a suitable solvent to form a patterned superconducting film, i.e. the first electrode 1. Photoresist is applied again on the substrate and the first electrode, and is exposed to light through another patterned mask and developed. After that, a part of the first electrode 1 is exposed to a water cleaning treatment. When the surface of the first electrode comes in contact with water during the above-mentioned water cleaning treatment and with the atmosphere, an undesired oxide film, of which the quality and thickness are not controlled, is formed on the first electrode 1. In order to remove the undesired oxide film from the first electrode 1, the exposed surface of the first electrode is subjected to sputter etching in an argon atmosphere. Immediately after the sputter etching, the exposed surface of the first electrode is oxidized by a thermal oxidation process, a dc oxygen glow discharge process or an rf oxygen glow discharge process to form the desired oxide film 3 serving as a tunneling barrier. Then, another superconducting material film is deposited on the remaining photoresist and the oxide film by the above-mentioned deposition method. The remaining photoresist and a part of the deposited film are removed at the same time by using a suitable solvent to form a patterned superconducting film, i.e. the second electrode 2. Thus, the Josephson element of the tunneling junction type is completed.
However, the above-mentioned sputter etching in an argon atmosphere often generates hillocks which protrude through the subsequently formed oxide film 3 and induce microshorts on the surface of the first electrode 1.